During high speed data transmission system components often radiate energy can couple or interfere with other system components and introduce signal corruption. Energy may also couple to sensitive circuits through other means. For example, energy may couple through ground or supply networks. The interference or “kick-back” caused by the coupled energy can cause problems in high speed data converters, such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACS).
FIG. 1 illustrates an exemplary digital signal and the corresponding interference or kickback. As seen in FIG. 1, when the digital signal is at a low-level, some relatively constant number of bits are toggling each cycle. The toggling could be caused, for example, by the energy coupling to the system. In this state, as seen in FIG. 1, the kickback is proportional to a thermal noise of the system. However, in the large signal condition, the digital signal “saturates” at full scale. That is, the digital representation of a signal at full scale is simply all 1's. In this condition, there is no kick-back, as no bits are toggling from cycle to cycle. Accordingly, in a conventional system, because the amount of kick-back is data dependent, there is a data-dependent signature in the output data.
In many systems, having a small amount of consistent noise is tolerable. For example, in audio or video systems, random noise is preferable to deterministic or data dependent-noise, because when noise is data-dependent, the noise becomes much more obvious.
Accordingly, there is a need to equalize the digital interference over all operating conditions during data transmission so that the interference is not data dependent.